If one's parents are sinners, how can that person obey them ? What are the boundaries of obeying parents ?
The commander of the faithful (P.B.U.H) has said : " Goodness towards (one's) parents is the greatest obligatory act." Mizanul Hikmah, Volume 10, Page 709
Imam al-Ridha(P.B.U.H) has said:" Allah, The Mighty, The Glorious, has commanded three things with which he has associated three other things. He associated payment of zakat along with prayers such that the one who offers prayers , but desists from offering zakat, his prayers are rejected. He associated gratitude to him with gratitude towards parents such that one who is grateful to Allah, but ungrateful to his parents , will be deemed as ungrateful to Allah. He associated righteousness with bonds of kinship such that one who is righteous, but has severed relations with his kin, will be deemed as not being righteous." Biharul Anwar, Volume 74, Page 77
so it is not our headic to concern of our parents sin and mis behave with them we should pray for them to ALLAH TO FORGIVE THEIR SIN
You need to be careful of this look that you have towards your parents that they're sinners and you're somehow an angel. I'm not saying you're a bad person, but we're all sinners, don't take the road of Iblis and think "I'm better than x".
That being said, while nothing in Quran or Sunnah explicitly says to obey your parents, as in to do everything they ask of you. However,
You always obey you parents to the extent that you do not disobey Allah or do injustice.
Even when you have made absolutely sure that you disobey them only for the sake of Allah, disobeying them doesn't mean treating them harshly or giving them words unbefitting of a child towards their parents. You continue to treat them with respect but explain to them your reasons kindly and ask Allah to forgive them.
Nice detailed answer here: https://www.islamicity.org/5823/say-not-even-fie-to-parents/
Do not obey them if they ask you to disobey Allah.